The present invention relates to high frequency power dividers and power combiners and, more specifically, to high frequency stripline and airstripline power dividers/combiners.
Stripline-type power dividers and power combiners are generally well known in the art of high frequency power manipulation (frequency range of approximately 2-18 GHz). Further, it is generally well-known in the art that such power dividers are structurally identical to power combiners. A power divider is typically formed as a patterned metal layer having an input power strip and two power output strips. The power combiner differs only in that the inputs and outputs are reversed so as to have two inputs and a single output. Thus, a power divider/combiner structure can, and will hereinafter, be generically referred to as a power divider. The particular design of the patterned metal layer of stripline-type power dividers is a product of well-known equations solved for conductors operating substantially in the TEM mode. The metal layer is usually supported by a dielectric substrate and further surrounded by a conductive ground plane.
The airstripline configuration power divider is similar to the other stripline-type power dividers in that it uses a patterned metal layer for the input power signal's conductive path. However, it differs in that a second metal layer, patterned as a mirror image of the first, is provided on a parallel opposing surface of the dielectric substrate and positioned so as to have a topological one-to-one correspondence. The ends of the respective input and output strips are then conductively connected to form a single, operative power divider having parallel conductive paths.
A well-known problem associated with the practical operation of power dividers is the need to effectively isolate each of the power inputs from any portion of the power output signal reflected back into the other power output of the divider. Reflection of a portion or all of the power output signal back into its respective power output may be caused by a mismatch in impedance or open circuit condition between the power output and its corresponding load device.
The necessary isolation is typically provided by connecting a resistive load between the output strips of the power divider. Given that the divider has a center operating frequency of: ##EQU1## where C is the speed of light in free space, .epsilon. is the relative dielectric constant, and .lambda. is the wavelength of the signal (f.sub.C thus being proportional to 1/.lambda.), the load resistance is connected at points a multiple of .lambda./4 distant from the junction of the power input strip and the power output strips. This provides a portion of the reflected power output signal with a conductive path between the power outputs that is approximately a distance of .lambda./2 shorter than the path traversed by the remainder of the reflected power output signal. This produces an approximately 180.degree. phase difference between the two portions of the reflected power output signal that, consequently, results in the effective cancellation of the reflected power output signal.
A particular problem in the efficient fabrication of high frequency power dividers is the need to physically place the resistive load between the output strips of the dividers. The resistive load is usually either a standard high frequency resistor whose leads have been soldered to the respective output strips or a discrete, chip-like, thin-film resistor which has been placed in a depression formed in the substrate and soldered between the two output strips. In either case, the requirement that the load resistance be physically placed and soldered into position comprises the simplicity and accuracy of the fabrication process which results in increased cost and decreased device yield.